1. Technical Field of the Invention
This invention relates to an apparatus for measuring in situ the viscosity of a fluid. More particularly, the present invention relates to an apparatus which allows the measurement of a useful parameter for measuring the viscosity of a fluid held within a sealed laboratory container wherein the measurement can be obtained without disturbing the fluid held therein.
2. Description of the Prior Art
The viscosity of a fluid is a measure of its "flow thickness." Viscosity measuring instruments include falling sphere viscometers (FSV) and rolling sphere viscometers (RSV) based on the Stoke's Law principle, as described in The Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 20, p. 299-300 (John Wiley 1982). Briefly, Stoke's Law relates the viscosity of a Newtonian fluid to the velocity of a falling sphere. When a sphere is allowed to fall freely within a fluid, it accelerates until the viscous force exactly balances the gravitational force. The Stokes equation for determining viscosity of a fluid using a falling sphere is: ##EQU1## where r is the radius of the sphere, d.sub.s and d.sub.l are the density of the sphere and the liquid, respectively, g is the gravitational force, and v is the velocity of the sphere. Falling sphere viscometers are generally limited to Newtonian fluids, although they are excellent instruments for routine monitoring of relative viscosities of various fluids.
A generalized form of the Stokes equation for a rolling sphere wherein the sphere rolls down the wall of a cylindrical tube is: ##EQU2## where v is the translational velocity of the rolling sphere and k is the instrument constant, which is determined by calibration with standard fluids.
In equations (1) and (2), v is the terminal velocity after the sphere has accelerated to maximum velocity, which is determined by measuring the elapsed time, t, for the sphere to fall or roll between two points separated by a known distance d. Assuming that the terminal velocity had been reached at the first point, v=d/t. For a FSV or RSV to be compact or about the size of typical laboratory instruments, it is necessary that v be relatively low such that an excessive initial fall or roll distance is not needed for the sphere to accelerate to v; and an excessive distance between the two measuring points is not required for the elapsed time to be measured accurately.
A viscosity measurement of a fluid with flow thickness similar to that of water by use of a laboratory size container would be very difficult to obtain using FSV or RSV as currently designed having spheres of glass or metal unless the sphere diameter was designed to be very small. Such a design would, however, produce handling problems and require extremely sensitive methods to detect the presence of the sphere at the two measuring points, described above, to reach desired accuracy.
Several types of rotational viscometers are well known, such as described in Viscosity and Flow Measurement, A Laboratory Handbook of Rheology, by J. R. Van Wazer, J. W. Lyons, K. Y. Kim and R. E. Colwell, Interscience Publishers (John Wiley & Sons, New York, 1963). In these models, the rotating member of desired geometric shape, which is essentially symmetrical, is mounted on a shaft and connected to the measuring instrument, usually positioned above the liquid to be measured. The rotating member is immersed in the liquid and operated at several constant speeds or rates of rotation. At each speed, the torque required to rotate the member at that rate is also measured. A measure of viscosity of the liquid may be calculated from the torque and the speed by appropriate mathematical relations, or the instrument can be calibrated by measuring the torque and speed of rotation for a number of liquids of known viscosity. The rotating member is usually in the shape of a disc, cylinder, cone, or sphere or some other symmetrical shape such as an inverted cup.
A significant problem exists for the FSV and RSV as well as for the rotating viscometers described immediately above. These instruments require that a sample be removed from a larger supply of the fluid to be tested which is contained in a separate container. This requires that the fluid be exposed to the atmosphere and often sophisticated equipment must be used to keep the sample from becoming contaminated since some fluids such as blood may undergo additional tests which require it to remain sterile. Therefore, a system is needed which facilitates a measurement of the viscosity of a fluid held in a laboratory container without the removal of the fluid therefrom.
Moreover, a spherical ball useful in RSV or FSV systems is needed which has a sufficient diameter to be detected by conventional sensing instruments but which is not too dense so that it can be used in conventional laboratory containers. Glass or metal balls, if shipped in a frangible laboratory container, would be likely to shatter the container during shipment unless they were formed to be extremely small and therefore difficult to detect.